Effects of climate change on the Mediterranean

Global change and microplankton

Microplankton diversity

Plankton is a collective term for all organisms living in the water column that lack their own means of active movement or whose range of movements are more or less negligible in comparison to the movement of the water mass as a whole. Plankton organisms can range in size from a few metres for large jellyfish and salp colonies to less than a micrometre for bacteria. Within the MarPLAN project the biodiversity of eukaryotic marine single-celled plankton organisms was studied in order to answer the question “In what ways can global change affect microplankton?”

To understand plankton distribution and changes therein, we first need to know how diverse it is. Diversity can be hidden within an easily identifiable morphologically defined species. Although this species may be considered cosmopolitan, it can possibly be divided into several separate species, or populations, each with a different distribution patter. For example, MarPLAN discovered that the cosmopolitan species Fibrocapsa japonica in fact consists of two different species. The second one was discovered in the Adriatic Sea.

Effects on phytoplankton

Ceratium sp.

In the temperate zones, many phytoplankton species form blooms during restricted periods of the year. Global warming caused some species to bloom earlier in certain places, and to shift the distribution of these blooms tends towards the poles. New species may appear in regions, partly through introduction (for example, via ballast water dumping) and partly through polewards range expansion of warm-water species.

Several MarPLAN researchers collaborated to assess these trends in the dinoflagellate genus Ceratium.

Over the last century, several Ceratium species have disappeared from study sites in Villefranche sur Mer and Naples, or have become far less common, while new dinoflagellate species have recently appeared.

Emiliania huxleyi.

Another driver of global change is the increased concentration of CO₂ in the atmosphere, which results in a higher CO₂ concentration in the upper layers of the ocean. This might seem a good thing for phytoplankton. However, there is a less favourable side-effect: with increasing CO₂ in the seawater, the acidity increases (the pH drops). As the acidity of seawater increases, it will be more difficult to produce the mineral calcium carbonate. This can cause problems for phytoplankton species that utilise calcium carbonate as a construction material for their cell walls. The coccolithophorid Emiliania huxleyi is one such species: it forms discs of calcium carbonate called coccoliths, which appear to provide protection to the cell.

Harmful phytoplankton blooms

Many phytoplankton species produce toxins or otherwise constitute a nuisance to other species, including humans (see also: here and here). Such species (for example, Fibrocapsa japonica) are harmful and, when they appear in large numbers, form harmful algal blooms (HABs). Global change may cause increasing numbers of HABs to appear in coastal regions.

Effects on zooplankton

The appearance of zooplankton (copepods, planktonic larvae of meiobethos) may be triggered by different factors: increased temperatures may affect the timing of appearance of certain species differently. If grazers such as planktonic larvae are out of phase with their food source they will starve and not make it into adulthood. Populations of benthic species which rely on zooplankton for nutrients may also decrease. These temporal changes, documented by DEEPSETS, have occurred within our lifetime.

Effects on the deep Mediterranean

DEEPSETS research has shown that the eastern Mediterranean is subjected to periodical events which deliver large amounts of food to the sea floor. These events abruptly turn the ‘desert’ into an ‘oasis’. This was illustrated by the very high phyto-pigment concentrations in the Ierapetra Basin during 1993. These were linked to an increased flow of nutrient-rich water into the Cretan Sea after 1992, which resulted in an enhanced biological productivity and organic matter flux to the seabed. In 1993, this enhanced flux caused significant changes in the abundance and composition of the meiobenthic assemblages as well as of the planktonic and macrobenthic communities.

Species detection tools

Reliable tools have been developed to detect declining species, isolated populations and exotic species. Climate change is thought to cause range shifts of marine fish species and local and global extinctions are predicted, although the latter is yet to be observed.

Small pelagic fish species, in particular, have large population sizes and a high potential for gene flow. Therefore, they may respond rapidly to changes in physical oceanographic conditions. They have, for instance, shown large population fluctuations and local extinctions over glacial time-scales.

Sprat and climate change

MarBEF presented a range-wide phylogeographic survey of European sprat Sprattus sprattus, based on a 530-base-pair sequence from mitochondrial DNA. This DNA region demonstrated the existence of genetically isolated populations in northern Mediterranean basins. MarBEF concluded that these populations, which have a significantly reduced genetic diversity, remain isolated because they can't maintain gene flow with other populations under the present physical oceanographic regime.

The results demonstrate the effect of past glacially-induced changes in physical oceanographic conditions on a cold-adapted small pelagic fish species. This species is now geographically isolated at its southernmost distribution limit, namely the Northern Mediterranean.